EP0058533A1

EP0058533A1 - Variable diffuser device in a centrifugal compressor

Info

Publication number: EP0058533A1
Application number: EP82300700A
Authority: EP
Inventors: Hiroshi Nagasaki Shipyard & Engine Works Nakatomi
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1981-02-16
Filing date: 1982-02-12
Publication date: 1982-08-25
Also published as: DK59482A; DK153244B; DK153244C; DE3264706D1; US4770605A; EP0058533B1

Abstract

A diffuser device (4') disposed in a passageway between an air outlet of an impeller (16) and a swirl chamber bounded by a volute casing (1,2) in a centrifugal compressor, comprises a diffuser ring (4) adapted to be fixed to a casing-wall defining the passageway, and a plurality of separate blades (9) for mounting on said ring at circumferentially spaced positions. Each diffuser blade have a boss (9a) formed at one common end dimensioned to fit in a respective hole (41) drilled in the diffuser ring, the holes 41 being equal in number to the blades and being arranged circumferentially around the ring at the spacings required for said blades, and said holes having their axes parallel to each other and to the axis of the ring. The bosses of said blades are freely rotatable about the axes of their respective holes, whereby the blade angle (a°) can be varied. The blades can then each be set at a desired blade angle by adjustment means (10, 12, 13 etc).

The invention provides a method for manufacturing such a diffuser device as well as a coupled drive mechanism (52, 52' 53) for the diffuser blades for rotating said blades in unison to a desired blade angle.

Description

The present invention relates to a centrifugal compressor of an exhaust gas turbine supercharger or the like, and more particularly to a diffuser device disposed in the passageway between the air outlet of the impeller and the swirl chamber within the casing of such a centrifugal compressor, and also to a method for manufacturing the diffuser device.
The essential parts of the compressor of one example of exhaust gas turbine supercharger known from the prior art are illustrated in cross-section in Figure 1. In the construction shown in Figure 1 an external supply of fresh air is drawn in by inducer blades 5 and an impeller mounted on a rotor shaft 7, which is driven by an exhaust gas turbine and imparts kinetic energy thereto. This kinetic energy in the air supply is converted into pressure energy by means of a diffuser device 4' and is supplied at the required pressure to a diesel engine through a swirl chamber formed by an outer volute casing 1 and an inner volute casing 2. The diffuser device 4' is securely fixed to the inner volute casing 2 by means of bolts 20. The air flow supplied by the supercharger is matched with the pressure and flow rate required by the diesel engine generally by means of the diffuser device 4', inducer blades 5 and impeller 6, but it is common practice to achieve fine adjustment of the flow parameters by means of the diffuser device 4'. Even with the aid of the latest sophisticated computing techniques it is not possible to match the precise flow requirements when using a diffuser device 4' of fixed construction as shown in Figure 1. It is usual, therefore, to prepare several slightly different diffuser devices and to choose that which tests show most closely conforms to the ideal requirement. However, the increase of the testing period, cost and amount of storage due to preparation of such diffuser devices raises the overall cost of a supercharger. Moreover, there is also the disadvantage that the preparation of two or more diffuser devices results in wasteful investment.
It is therefore one object of the present invention to provide a diffuser device in a centrifugal compressor which can achieve the required matching of pressure and flow rate with only one variable construction.
Another object of the present invention is to provide a method for manufacturing a diffuser device in a centrifugal compressor, in which an adjustable diffuser device can be easily and accurately assembled, and which method is suitable for mass-production at low cost.
A further object of the present invention is to provide an adjustable diffuser device in a centrifugal compressor, which includes coupled drive means capable of simultaneously and accurately adjusting a plurality of diffuser blades.
According to one feature of the present invention, there is provided a diffuser device disposed in the passageway between the air outlet from the impeller and the swirl chamber within the casing of a centrifugal compressor, which diffuser device comprises a diffuser ring adapted to be fixed to the casing, and a plurality of separate diffuser blades intended to be spaced circumferentially around the diffuser ring, and in which each diffuser blade has one end formed to fit into a respective hole circumferentially spaced around the diffuser ring, each blade being freely rotatable with respect to its hole, whereby the blades can be rotated to a desired angle, and set at such angle.
According to another feature of the present invention, there is provided a method for manufacturing the diffuser device of a centrifugal compressor which consists of providing separately a diffuser ring and a plurality of diffuser blades, forming a cylindrical boss at one common end of the diffuser blades, forming a hole for receiving a said boss at appropriate locations circumferentially around the diffuser ring, fitting the boss of each diffuser blade into its respective hole in the diffuser ring, adjusting the inlet and outlet 'apertures between adjacent diffuser blades by rotationally positioning the latter, and fixing the diffuser blades in the required rotational position with respect to the diffuser ring.
In order that the invention may be readily understood and further features made apparent, several embodiments thereof will now be described with reference to the accompanying drawings in which :

Figure 1 is a cross-sectional view showing the essential parts of the compressor of one example of exhaust gas turbine supercharger of the prior art.
Figure 2(a)is across -sectional side view showing the essential parts of the compressor of an exhaust gas turbine supercharger according to one preferred embodiment of the present invention,
Figure 2(b) is a cross-sectional plan view taken along line A-A in Figure 2(a) as viewed in the direction of the arrows,
Figure 3(a) is an enlarged cross-sectional view showing the details of the mounting of an adjustment flange and diffuser blades onto a diffuser ring generally illustrated in Figures 2(a) and 2(b),
Figure 3(b) is a plan view of that part of the diffuser device illustrated in Figure 3(a),
Figure 4(a) is a cross-sectional side view showing a diffuser device according to another preferred embodiment of the present invention,
Figure 4(b) is a plan view of the diffuser device illustrated in Figure 4(a),
Figure 4(c) is an enlarged side view of a diffuser blade in the same diffuser device,
Figure 4(d) is a cross-sectionalplan view of the diffuser blade taken along line B-B in Figure 4(c) as viewed in the direction of the arrows,
Figure 5(a) is a cross-sectional side view showing a diffuser device according to a further preferred embodiment of the present invention, in which a coupled drive mechanism for simultaneously adjusting every diffuser blade is included, _i
Figure 5(b) is a cross-sectional plan view taken along line Y-Y in Figure 5(a) as viewed in the direction of the arrows,
Figure 6 (a) is a schematic view showing the geometrical relationship between two adjacent sprockets in the preferred embodiment illustrated in Figures 5(a) and 5(b),
Figure 6(b) is a schematic view showing the geometrical relationship between sprockets and a roller chain in the same preferred embodiment,
Figure 6 (c) is a cross-sectional side view taken along line X-X in Figure 6 (b) as viewed in the direction of the arrows,
Figure 7 (a) is a schematic view showing the geometrical relationship between two adjacent sprockets in a coupled drive mechanism for diffuser blades according to yet another preferred embodiment of the present invention, !
Figure 7 (b) is a schematic view showing the geometrical relationship between sprockets and roller chains in the preferred embodiment illustrated in Figure 7 (a), and,
Figure 7 (c) is a cross-sectional side view taken along line X' - X' in Figure 7 (b) as viewed in the direction of the arrows.

The present invention is generally applicable to any centrifugal compressor such as an air compressor, a gas turbine, an exhaust gas turbine supercharger, a gas compressor, a centrifugal pump, etc., but in the following, for the sake of convenience, it will be described in more detail in connection with its preferred embodiments as applied to an exhaust gas turbine supercharger.
In one preferred embodiment illustrated in Figures 2(a), 2(b), 3(a) and 3(b) like or equivalent parts to those described hereinbefore with respect to the prior art construction shown in Figure 1 are given like reference numerals. In this embodiment, the diffuser device 4' provided in the passageway between the impeller 6 and the swirl chamber bounded by the outer volute casing 1 and the inner volute casing 2 is formed as a separate diffuser ring 4 mounted on one side of the passageway, and separate diffuser blades 9 adapted to be connected to the diffuser ring 4.
Each diffuser blade 9 is provided with a boss 9a below its leading edge, and (see particularly Figure 3(a)) the boss 9a has a positioning pin 13 securely fixed thereto and a threaded hole formed therein such that a central bolt 12 may be screwed into the hole via a spring washer 15.
An adjustment flange 10 is provided with a similar boss having the same diameter as the above-described boss, and this boss has an appropriate number of positioning holes 13a drilled in it, so that the above-mentioned positioning pin 13 may be selectively inserted therein, at predetermined intervals circumferentially around the adjustment flange 10, and also at the centre of the said boss a bolt hole is provided that is coaxial with the threaded hole in the above-described boss 9a. Other holes are drilled in the peripheral portion of the adjustment flange 10 for receiving fixing bolts 11 and parallel pins 14 to be used for relative positioning between the adjustment flange 10 and the diffuser ring 4.
A hole 41 is drilled through the diffuser ring 4, into which the boss 9a of the diffuser blade 9 and the similar boss of the adjustment flange 10 are inserted opposite each other. Then, by inserting the positioning pin 13 into a selected positioning hole 13a and tightening the bolts 11 and 12 to fix the relative positions of the adjustment flange 10, diffuser blade 9 and diffuser ring 4, the diffuser blades 9 can be mounted on the diffuser ring 4 at a predetermined mounting angle, so as to have a predetermined inlet angler , inlet aperture a and outlet aperture b as indicated in Figures 2(b) and 3(b).
An O-ring 8 is provided in intimate contact with the diffuser ring 4 on the side exposed to the thrust in order to prevent leakage of compressed air and vibration of the diffuser blades 9.
The mode of operation of the diffuser device constructed as described above is as follows :

A fresh supply of air is drawn in through the inlet casing 3, by means of the inducer blades 5 and kinetic energy is imparted thereto by the impeller 6 and rotor shaft 7. This kinetic energy is converted to pressure energy by means of the diffuser blades 9 attached to the diffuser ring 4. Thereafter, the compressed air is supplied at the required pressure and flow rate to a diesel engine through the swirl chamber bounded by the outer volute casing 1 and the inner volute casing 2.

In the event that it becomes necessary to change the specification of the diffuser, a change of the inlet angle α°, inlet aperture a and outlet aperture b can be effected in a simple manner by removing the inlet casing 3, drawing out the inner volute casing 2 a distance of about 5 - 10 mm, removing the bolts 11 and 12 and the spring washer 15 which fix the adjusting flange 10, extracting the positioning pin 13 from the positioning hole 13a located at reference index point O for the inlet angle α° of the diffuser blade 9 and inserting it into another positioning hole 13a located at an adjacent index point x 1 or - 1, while slightly separating the adjustment flange 10 from the boss 9a, and then re-assembling the diffuser device in the sequence opposite to that mentioned above. In this case, the inner diameter of the array of diffuser blades 9 serves as a datum for every adjustment.
As will be seen from Figure 2(b), the pitch 8 of the diffuser blades 9 is 8 = 360⁰/Z, where Z represents the number of diffuser blades 9. In the case where the required angle of the diffuser blades 9 cannot be achieved by means of adjusting the positioning pin 13 relative to the positioning holes 13a, instead of replacing the diffuser blades 9, adjustment flanges 10 having a different reference value of 0 the inlet angle α could be prepared and substituted for the previous ones. Then the diffuser device can be reassembled in a simple manner so as to realize any desired blade angle.
Such a construction of diffuser device is particularly useful where it becomes necessary to change the specification of a diffuser device in order to match the requirements of a diesel engine more exactly with an exhaust gas turbine supercharger; the necessary specification of the diffuser device can be realized simply and economically by replacing cheap adjustment flanges and/or selecting new positioning holes, instead of replacing the entire diffuser device, or the expensive diffuser blades. Thus, the advantages of the construction can be summarised as follows

1) The cost is low.
2) A change of specification of the diffuser can be effected in a simple manner by relatively minor adjustments to the exhaust gas turbine supercharger on a diesel engine.
3) The adjustment flange 10 can be re-used.
4) The inlet angle α°, inlet aperture a and outlet aperture b of the diffuser blades 9 can be selectively varied by means of the adjustment flange 10.
5) Adjustment of the diffuser device can be effected without removing it from the exhaust gas turbine supercharger.

It is to be noted that while the bosses are provided at the leading edges of the diffuser blades in the above-described embodiment, it is also. possible to position the bosses anywhere along the length of the diffuser blades and to effect adjustment in a similar manner to that described above by taking the diameter through the selected positions in the array of diffuser blades as a datum. (However, due to interference with the inner volute casing 2, depending upon the selected datum diameter sometimes it may happen that a change of specification cannot be effected unless the diffuser device is removed).
In another preferred embodiment as shown in Figures 4(a) to 4(d) the blade angle of the diffuser blades, and consequently the inlet and outlet apertures between the diffuser blades, is made continuously adjustable without employing the positioning pin 13 as used in the preceding embodiment. Thus, in Figures 4(a) to 4(d), a diffuser ring has a number of holes 41 of diameter d drilled at equal intervals adjacent to its inner diameter there being as many holes 41 as there are blades 9 (z). Each diffuser blade 9 is of aero-foil cross-section and is integral with a cylindrical boss 9a of diameter d centered at its leading edge, and it may be produced by precision casting or precision forging.
With regard to the configuration of the blade 9, a diffuser device having a low pressure ratio can have blades of rectilinear cross-section, whilst a diffuser device having a higher pressure ratio must have blades of aero-foil cross-section to give a good efficiency.
In order to assemble the complete diffuser, the bosses 9a of the diffuser blades 9 are inserted into the corresponding holes 41 of diameter d in the diffuser ring 4, the desired inlet aperture a and outlet aperture b are preset by transversely moving the trailing edges of the diffuser blades 9, then, while holding them in this position by means of jigs, which prevent further movement, brazing is effected between the diffuser ring 4 and the diffuser blades 9 and between the holes 41 and the bosses 9a within a vacuum furnace, or within an argon atmosphere to bond them together; thereby any desired specification of the diffuser device can be realized.
In summary, a diffuser device 4' is made up of a separate diffuser ring 4 and a number of diffuser blades 9, and after the desired inlet and outlet angles and inlet and outlet apertures have been preset by means of positioning jigs while transversely moving the trailing edges of the diffuser blades, the diffuser blades 9 are bonded to the diffuser ring 4. With the construction of this embodiment, because the ring of drilled holes can be accurately positioned in the diffuser ring, and cylindrical bosses formed at one end of the diffuser blades are fitted into these holes, the following advantages can be obtained:-

a) Diffuser blades can be preset precisely at equal intervals.
b) Adjustment of the inlet and outlet apertures can be achieved easily by merely moving the trailing edges of diffuser blades.
c) A boss formed integrally with a diffuser blade can serve as a reinforcement for the diffuser blade (especially at its leading edge where it is vulnerable), and it also makes positioning of the diffuser blade easy.
d) Rigid mounting of a diffuser blade onto a diffuser ring is facilitated, and also there is no danger that the diffuser blade may tilt during brazing.
e) Inter-changeability of diffuser blades is enhanced and as a result, diffuser blades can be mass-produced economically and can be quickly assembled into complete diffusers.
f) With regard to the setting angle of the diffuser blades, it is not always possible to obtain the optimum result by exactly reproducing the calculated design angle, and in the event that one fails to obtain the optimum result, the diffuser device according to the above-described embodiment is especially suitable for determining the optimum setting angle experimentally by varying the inlet and outlet angles and inlet and outlet apertures.

It is to be noted that although the size of the boss is limited by the radial depth of the diffuser ring, and the space between the holes, for the purpose of fully realizing the above-mentioned advantages, it is preferable to select the diameter of the boss to be as large as possible. However, since the ring itself does not require very great mechanical strength, with respect to this point there is no need to impose any restriction.
With regard to the mode of bonding the diffuser blades to a diffuser ring, possible alternative methods are as follows :-

1) In the case where the inner diameter D_l (Figure 4 (b)) of an array of diffuser blades is employed as a datum:

The holes 41 for mounting the diffuser blades 9 are drilled adjacent to the inner diameter of a diffuser ring 4, and bosses 9a integral with the diffuser blades 9 are centered at the leading edges of the blades.

2) In the case where the outer diameter D₂ (Figure 4(b)) of an array of diffuser blades is employed as a datum:

The holes 42 for mounting the diffuser blades 9 are drilled adjacent to the outer diameter of a diffuser ring 4, and bosses 9a integral with the diffuser blades 9 are centered at the trailing edges of the blades.

3) In the case where an arbitrary diameter in an array of diffuser blades is employed as a datum:

The holes for mounting the diffuser blades are drilled around the circle of arbitrary diameter in a diffuser ring 4, and bosses 9a integral with the diffuser blades 9 are positioned at arbitrary points on the blades.
In every case, the inlet aperture a and outlet aperture b of the diffuser blades are simultaneously adjusted with positioning jigs while transversely moving the diffuser blades about their bosses, and then the diffuser blades are held in place and bonded to the diffuser blades by brazing.
As described above, whilst the relative positions of a diffuser blade and its boss may be arbitrarily chosen, for ease of adjustment of the mounting angle and greatest reinforcing effect, the position 1) above is most preferable.
Summarizing the method for manufacturing a diffuser device according to the above-described embodiment, it consists of the following four steps:

1) A cylindrical boss is made integral with a diffuser blade and centered at the leading (or trailing)edge of the diffuser blade at its inlet (or outlet) edge.
2) Holes are drilled equally spaced in a diffuser ring by employing a blade inlet (or a blade outlet) as a reference to be used as a reference for mounting diffuser blades.
3) Diffuser blades are produced by precision casting (or precision forging).
4) Diffuser blades are brazed to a diffuser ring (within a vacuum furnace or within an argon atmosphere).

As a result of such construction and/or such method of manufacture of a diffuser device, the following advantages can be obtained:

1) In the case where a boss is mounted at the inlet edge, the structure can well withstand a high frequency vibration induced by an impeller (because the blade and the boss are integrally fixed to the diffuser ring).
2) The rate of mass-production can be high, and hence the manufacturing cost can be reduced.
3) Regardless of respective specifications, diffuser blades and-diffuser rings of a single type (i.e. unit construction) can be used in common.
4) The period required for manufacture can be shortened (from 3 - 4 months to 10 - 15 days).

Now two other preferred embodiments of the present invention, in which adjustment of the diffuser blades in a diffuser device is effected by means of a coupled drive mechanism which enables all the diffuser blades to be rotated simultaneously in the same phase, will be explained with reference to Figures 5(a), 5(b), 6(a), 6(b), and 6(c) and Figures 7(a), 7 (b) and 7 (c) respectively.
In Figures 5(a), 5(b), 6(a), 6(b) and 6(c), reference numeral 51 designates diffuser blades provided in a gas flow path of a compressor, numeral 52 designates a sprocket mounted on a rotary shaft of the diffuser blades.51 extending through an inner volute casing 59, numeral 53 designates a roller chain wound around a plurality of sprockets 52, and numeral 55 designates a diffuser ring which is interposed between the diffuser blades 51 and the surface of the inner volute casing 59 so as to be displaceable in the direction of the rotary shaft of the diffuser blades 51. Reference numeral 54 designates a spring-loaded cylinder which either urges the diffuser ring 55 towards the inner volute casing 59 or urges it away from the latter. Reference numeral 58 designates an outer volute casing, and reference numeral 60 designates an 0-ring provided for the purpose of preventing pressurized gas from escaping through the clearance between the back face of the diffuser ring 55 and the inner volute casing. Reference numeral 61 designates a shaft for externally driving the sprockets 52'. It is to be noted that the drive to the diffuser blades 51 could equally well be disposed on another casing 57 on the opposite side. Reference numeral 56 designates a compressor impeller. The diffuser device constructed in the above-described manner operates as follows. By supplying the spring-loaded cylinder 54 with compressed air or hydraulic pressure, the diffuser ring 55 is urged towards the inner volute casing 59 by an amount C to form gap clearances C_l and C₂, respectively, on either side of the diffuser blades 51. In this condition, the drive shaft 61, carrying the driving sprocket 52' engaging the roller chain 53, rotates and thereby causes adjustment to be made to the diffuser blades 51 by means of their coupled sprockets 52 also engaging with the roller chain 53 so that the diffuser blades 51 are set to the necessary inlet angle/3 and desired inlet aperture a, and thus the required specification of the diffuser device can be realized.
With regard to the change of the blade angle by employing the inner diameter of the blade array as a datum, description will be made with reference to Figure 6(a), 6(b), and 6(c). In these figures, representing the number of the diffuser blades 51 by Zn, the centre of the diffuser blade array by 0, the centres of rotation for adjustment of the respective diffuser blades by 0₁, 0₂,........On, the central angle of the arc 0₁0₂ by α = 360°/Zn =< 0₁00₂, the radius of the sprockets for the respective diffuser blades by R, and the diameter of the circle passing through the centres 0₁, 0₂, ........O_n by D, then the amount of movement of every point on the respective sprockets 52 in the case of driving the sprockets for the respective diffuser blades by stretching a roller chain 53 around the respective sprockets is calculated as follows :

(1) Common tangents are drawn for two circles of diameter R representing the sprockets having two adjacent centres (O_n-0₁, 0₁- 0₂, ⁰ ₂ -0₃, etc.), and the common points between the circles 0₁, 0₂, ..... and the common tangents are designated by A', A", B', B",....
(2) An intersection between a common tangent for two adjacent circles representing sprockets and a bisector of a central angleα of a regular n-angle polygon determined by the number of blades Zn, is denoted by H. Then, the equation
is satisfied.

Then from trigonometrical considerations and the above assumptions (1) and (2), the following relationships are derived.

hence,we obtain
Therefore, when a point A" on a circle of a radius R and having its centre at 0₁ is moved to a point A, a point B on a circle of radius R and having its centre at 0₂ which circle represents a sprocket coupled via a roller chain to the sprocket represented by the former circle 0₁ is moved exactly to the point B'.
The diffuser device according to the above-described embodiment of the present invention provides the following advantages.

(1) The clearances C₁ and C₂ on either side of the diffuser blades can be varied between the period when the diffuser device is in use and the period when the inlet angle β is being varied. Therefore, in use the diffuser device has very small clearances near to zero; hence the performance is improved and the amplitude of vibration is small.
(2) By initially selecting the clearance C to its maximum allowable value for the performance, the torque required to vary the inlet angle β of the diffuser blades is as small as possible because the clearances C₁ and C₂ are a maximum.
(3) Since commercially available standard parts can be used, and since the number of parts is reduced, the cost of the diffuser device is lowered.
(4) The period required for manufacture is shortened because of the use of commercially available parts.
(5) If adjustable pieces are used in the roller chain, fine adjustment of the diffuser blades during assembly can be made.

Figures 7(a), 7(b) and 7(c) show a modified form of the preceding embodiment, to cover the case where the number of the diffuser blades 51 is so large that adjacent sprockets would interfere with each other if the preceding embodiment is employed. In this modified embodiment the axial positions of the sprockets for the respective diffuser blades are alternately varied, and the respective groups of sprockets are coupled with two separate loops of roller chain 53, and 53' for which two separate drive sprockets 52' and 52" are provided so as to turn the respective groups of sprockets through the same angle. The effects and advantages of the diffuser device according to this embodiment are exactly the same as those of the preceding embodiment.

Claims

1. A diffuser device disposed in a passageway between the outlet of an impeller and a swirl chamber within the casing of a centrifugal compressor, characterized in that said diffuser device (4') comprises a diffuser ring (4) adapted to be fixed to the casing (1,2) and a plurality of separate diffuser blades (9) intended to be spaced circumferentially around the diffuser ring, and in that each diffuser blade has one end (9a) formed so as to fit into a hole (41)provided in said diffuser ring, there being a hole for each diffuser blade and the holes being provided circumferentially around the diffuser ring with their axes parallel to each other and to the axis of said ring at the required spacing for the diffuser blades, and in that said one end of the diffuser blades are made to be freely rotatable about the axes of their respective holes, whereby said blades can be rotated to a desired blade angle (αC°).

2. A diffuser device as claimed in Claim 1, characterized in that adjustment means (10, 12, 13 etc) are provided for fixing respective diffuser blades in position relative to said diffuser ring, after the diffuser blades have been rotated to their desired blade angle.

₃. A diffuser device as claimed in Claim 1 or 2, characterized in that said diffuser ring (55-See Figures 5) is mounted with predetermined gap clearances (C_I and C₂) retained from the side surface of the casing (59) and from the side surfaces of the diffuser blades (51) so that it can be axially moved with respect to the casing, and said diffuser device comprises a ring drive mechanism (54) for reciprocating said diffuser ring in the axial direction and a diffuser blade drive mechanism (52, 52', 53), for rotationally driving said diffuser blades about the axes of their respective holes in the diffuser ring.

4. A diffuser device as claimed in Claim 3, characterized in that said ring drive mechanism consists of a hydraulic cylinder (54) having its piston fixedly secured to said diffuser ring, and said diffuser blade drive mechanism consists of sprockets (52) fixedly secured to respective diffuser blades, a roller chain (53) drivingly connected said sprockets, and a drive source for driving said chain.

5. A method for manufacturing a diffuser device in a centrifugal compressor as claimed in any preceding Claim, ; characterized by the steps of providing separately a diffuser ring and a plurality of diffuser blades, forming a cylindrical boss at one common end of said diffuser blades, forming holes for receiving said bosses at appropriate spacing circumferentially around the diffuser ring with the axes of said holes parallel to each other and to the axis of said ring, the number of holes being equal to the number of blades,fitting the bosses of said respective diffuser blades into respective holes in said diffuser ring, adjusting the inlet and outlet apertures between adjacent diffuser blades by rotationally positioning the latter, and fixing said diffuser blades in the desired rotational position with respect to said diffuser ring.